1. Field of the Invention
The invention described herein pertains to communications systems, and more particularly to physical layer parameter changes.
2. Background Art
In modem digital communications systems, communicating entities need to have a common, predetermined set of protocols and parameters. Given these protocols and parameters, the entities can communicate in an orderly, efficient manner. Such protocols and parameters are typically implemented according to general functionality. The various functions are often collectively modeled as multiple layers of a protocol stack. Each layer represents additional protocols that a communicating entity must process, and/or parameters that must be adopted. The lowest layer in the protocol stack is typically the physical layer. The physical layer establishes fundamental parameters relating to the format of signals over a physical medium. These parameters can include, for example, the modulation method to be used, the error detection and correction method, the number of symbols to be transmitted per second, the number of bits that are represented by each symbol, and, if bandwidth is allocated in terms of time slots, the slot size. In the context of a burst communications system, such parameters collectively represent a burst profile.
One example of a communications system standard that specifies a physical layer is the Data Over Cable System Interface Specification (DOCSIS). DOCSIS was originally conceived for cable communications systems. While DOCSIS can be applied to such communications systems, it is not necessarily limited to cable. Wireless communications systems, for example, can also operate under DOCSIS. Likewise, DOCSIS can be used in satellite communications systems.
DOCSIS can be used in communication systems that include a set of remote communications devices connected to a headend device, such that the headend is responsible for the management of communications both to and from the remotes. The headend is responsible for the distribution of information content to the remotes (the so-called “downstream” direction); in addition, the headend is responsible for management of communications in the other direction, from the remotes to the headend (the “upstream” direction). Generally, in addition to sending content to remotes, the headend issues downstream messages that instruct each remote as to when it can transmit upstream, and what kind of information it can send. In effect, the upstream bandwidth is controlled and allocated by the headend. Any given remote can transmit upstream only after requesting bandwidth and receiving a grant of the bandwidth from the headend. In a time division multiple access (TDMA) environment, bandwidth corresponds to one or more intervals of time. Moreover, the upstream can be organized into a number of channels, with several remotes assigned to each channel. This arrangement allows the headend to manage each upstream communications channel. In this manner, upstream communications are managed so as to maintain order and efficiency and, consequently, an adequate level of service.
In the realm of cable communications, DOCSIS specifies the requirements for interactions between a cable headend and associated remote cable modems. A cable headend is also known as a cable modem termination system (CMTS). DOCSIS consists of a group of specifications that cover operations support systems, management, and data interfaces, as well as network layer, data link layer, and physical layer transport. Note that DOCSIS does not specify an application layer. The DOCSIS specification includes extensive media access layer and physical (PHY) layer upstream parameter control for robustness and adaptability. DOCSIS also provides link layer security with authentication. This prevents theft of service and some assurance of traffic integrity.
The current version of DOCSIS (DOCSIS 1.1) uses a request/grant mechanism for allowing remote devices (such as cable modems) to access upstream bandwidth. DOCSIS 1.1 also allows the provision of different services to different parties who may be tied to a single modem. With respect to the processing of packets, DOCSIS 1.1 allows segmentation of large packets, which simplifies bandwidth allocation. DOCSIS 1.1 also allows for the combining of multiple small packets to increase throughput as necessary. Security features are present through the specification of 56-bit Data Encryption Standard (DES) encryption and decryption, to secure the privacy of a connection. DES is also used for authentication. DOCSIS 1.1 also provides for payload header suppression, whereby repetitive ethernet/IP header information can be suppressed for improved bandwidth utilization. DOCSIS 1.1 also supports dynamic channel change. Either or both of the downstream and upstream channels can be changed on the fly. This allows for load balancing of channels, which can improve robustness.
Sometimes it may be necessary to change the PHY parameters in a communications system. For example, user requirements may change such that a different symbol rate is needed. PHY parameters may also have to be changed as a result of changes in the communications environment. For example, if the communications environment becomes noisy, a different method of error correction coding may be required.
DOCSIS provides a method in which PHY parameters (i.e, a burst profile) can be changed. Such a change requires a reprogramming of components that handle PHY processing, including PHY devices at the headend. The parameter change process for headend PHY devices is illustrated generally in FIG. 1. The process as illustrated pertains to changing PHY parameters for upstream communications. The process starts with step 105. In step 110, the new PHY parameters for a given upstream channel are determined. In step 115, an upstream channel descriptor (UCD) is formulated. The UCD is a message sent from the headend to remote devices and contains the new PHY parameter values. In step 120, the UCD is sent downstream. In step 125, a determination is made as to the point in the upstream at which the new parameters are to take effect. In step 130, a downstream MAP message is formulated stating when, in the upstream, the change is to occur. Note that such a message is commonly denoted in capitalized form, “MAP”; this convention is used hereinafter. The role of MAP messages, generally, is to manage the upstream transmissions of remote devices. Such a message typically allocates, i.e., maps, specific time intervals in the upstream to specific remote devices, thereby allowing a given remote device to transmit upstream only in a specified time interval.
Note that upstream time intervals are defined based on a clock having a predetermined frequency, such as 10.24 MHz. Such a clock can, in some systems, be interpreted in terms of time units, or “ticks.” Each tick can, for example, be 6.25 microseconds. Ticks can be further organized into larger units called minislots. The number of ticks per minislot can be defined at the discretion of the headend. The available upstream bandwidth can therefore be viewed as a series of minislots. Moreover, MAP messages allocate the upstream bandwidth in terms of minislots.
In the case of changing PHY parameters in DOCSIS, a specific time interval (i.e., minislot sequence) is identified in which all remotes are barred from transmitting upstream. This is the interval in which reprogramming of the PHY devices with the new parameters is to take place. Because no remote devices are allowed to transmit during this interval, the interval is referred to as “dead time.” DOCSIS specifies that the dead time last one millisecond.
Returning to FIG. 1, in step 135, the MAP message is sent downstream. In step 140, the changeover point arrives (i.e., the start of first minislot of the dead time, as specified in the MAP message) and a central processing unit (CPU) at the headend is interrupted. This interrupt must be handled during the dead time. In step 145, the new parameters are written, via a port of the CPU, to the headend PHY device. The write process is driven by software executing on the CPU. The process concludes at step 150.
The method of FIG. 1 however places a significant burden on the software executing on the CPU of the headend. Within the dead time interval, the software receives the interrupt, must process the interrupt immediately and write the new parameters to the local PHY devices. Typically, the write process is performed by the CPU via a relatively slow serial interface. The write process can take up to six hundred microseconds. Therefore, to complete processing within a one millisecond dead time interval can be a challenge. Moreover, if the dead time is exceeded, remote devices may begin transmitting, using the new PHY parameters, before the headend is ready. As a result, upstream data may be lost. Hence there is a need for a system and method that allows efficient reprogramming of PHY devices at the headend, such that there is minimal risk of exceeding the dead time.